Purchase Offer With Due Diligence Contingency

[William Coet]

Hello,

I want to begin making offers on land and buildings and include a due diligence period.

When making offers on land or commercial property is it necessary to place a non-refundable deposit in order to include a due diligence period? 

What is the typical due diligence period for land when buying land that is intended for residential development and zoning approvals are needed.

Thanks!